Polyhedron 27 (2008) 3567–3574

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Polyhedron

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Structural aspects of the anti-cancer drug oxaliplatin: A combined theoretical andexperimental study

Prateek Tyagi, Pragya Gahlot, Rita Kakkar *

Department of Chemistry, University of Delhi, Delhi 110 007, India

a r t i c l e i n f o

Article history:Received 10 June 2008Accepted 26 August 2008Available online 10 October 2008

Keywords:OxaliplatinDFTFT-IRAnti-cancerConformation

0277-5387/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.poly.2008.08.025

* Corresponding author. Tel.: +91 1127666313.E-mail addresses: rkakkar@chemistry.du.ac.in, rita_

a b s t r a c t

The conformational behavior of the third generation antitumor drug, oxaliplatin, has been explored byGGA-PW91 density functional calculations and FT-IR spectra. The difference in the biological activitiesof cisplatin and oxaliplatin are attributed to the presence of the DACH ligand in the latter. The trans formsof the ligand are found to be more stable than the cis form, but, of the two equally stable enantiomers, thetrans-l (1R,2R) one is found to be more potent biologically. Since very minor differences are observed inthe electronic structures of the two enantiomers, their difference in activity is attributed to the chiral rec-ognition of the ligand by DNA. The calculated vibrational frequencies are in good agreement with ourexperimental FT-IR spectrum. Calculations have also been performed on the cis isomer and its monohy-drate. Comparison between the theoretically predicted geometries and the experimental ones yieldedgood correspondence, validating our methodology.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Since the accidental discovery of the biological activity of theplatinum complex, cisplatin, in 1965 by Rosenberg [1], a largenumber of platinum complexes have been synthesized for im-proved pharmacological properties. However, only a few of thesecompounds have entered clinical trials and very few have been ap-proved for cancer therapy.

Although cisplatin (cis-dichlorodiammineplatinum(II)), [cis-(NH3)2PtCl2]), has found widespread use as an antitumor drug inthe past 40 years [2,3], it has many drawbacks associated withits use, the main being drug resistance. Moreover, it is ineffectivefor some cancers, and has major toxic limitations, of which neph-rotoxicity is the most notable. Nausea and vomiting, peripheralneuropathy, and cytotoxicity are also some of the major side ef-fects of this drug [4–6]. And so the quest began for cisplatin ana-logues that are more potent and effective against a larger rangeof tumors, are less toxic, have fewer side effects, and are not sub-ject to drug resistance. Structure–activity relationships (SAR) ledto the following general rules [7,8]:

(1) The general formulae should be cis-Pt(II)X2(N)2 and cis-Pt(IV)Y2X2(N)2 (where N: amine ligand, X: leaving group, Y:axial group). For the Pt(IV) compounds, the two Y ligandsshould be in trans orientation.

ll rights reserved.

kakkar@vsnl.com (R. Kakkar).

(2) The leaving ligands, usually anions, should consist of groupsthat have intermediate binding strength to Pt(II). Examplesof good leaving groups are Cl�, SO4

2�, citrate, oxalate andother carboxylic acid residues.

(3) The amine ligands, either monodentate or bidentate, shouldhave at least one NH group.

However, in recent years, a few unconventional platinum drugs,whose mechanism of action is different from that of cisplatin, haveemerged [9]. The SAR strategy has led to the ‘‘second generation”cisplatin analogue, carboplatin, cis-diammine(cyclobutane-1,1-dicarboxylato)platinum(II). Carboplatin, which was approved bythe FDA for the treatment of ovarian cancers in 1989, is less toxicthan the ‘‘first generation” antitumor drug, cisplatin. However, itis effective against the same range of tumors as cisplatin.

Of the ‘‘third generation” platinum analogues, compounds con-taining a 1,2-diaminocyclohexane (DACH) carrier ligand [10], suchas oxaliplatin [(1R,2R)-diaminocyclohexane(ethanedioate-O,O)platinum(II)] [10–13], have consistently demonstrated antitu-mor activity in cell lines with acquired cisplatin resistance and ap-pear to be active in tumor types that are intrinsically resistant tocisplatin and carboplatin [12–17].

In the present work, we have focused on oxaliplatin. The non-hydrolyzable 1,2-trans-R,R-diaminocyclohexane (DACH) ligandmakes the complex less polar, which contributes to better cell up-take [18]. The different antitumor and other biological effects ofoxaliplatin in comparison with those of conventional cisplatinare often explained by the ability of oxaliplatin to form DNA ad-ducts of different conformation and consequently to exhibit differ-

3568 P. Tyagi et al. / Polyhedron 27 (2008) 3567–3574

ent cytotoxic effects. Although differences between oxaliplatin andcisplatin in DNA binding, adduct formation, strand breaks, andapoptosis have been reported, the mechanisms behind the morepotent cytotoxic activity of oxaliplatin compared with cisplatinagainst colon cancer cells have not yet been completely elucidated.

On the theoretical front, the interaction of cisplatin with waterhas been studied by various authors [19–35]. However, to ourknowledge, no such studies have been reported for the third gen-eration platinum drug, oxaliplatin (eloxatin). In view of the biolog-ical importance of this compound, we have carried out asystematic study of the structural properties of oxaliplatin andits various conformational forms. After comparing the relativeenergies of various conformers, we identified the most stableone. To check the accuracy of the methods used, the results ofour density functional (DF) calculations were also compared withthe experimental results, wherever possible. In addition, the exper-imental FT-IR spectrum was determined for the more potent iso-mer and compared with the theoretically predicted values.

2. Methods

2.1. Computational details

First-principles density functional (DF) calculations were per-formed on the various conformers of oxaliplatin using the DMol3

code [36–39] available from Accelrys Inc. in the MATERIALS STUDIO 3.2package. Our calculations employed numerical basis sets of dou-ble-f quality plus polarization functions (DNP) to describe thevalence orbitals. This is the numerical equivalent of the Gaussian6-31G** basis set. The core electrons were treated using DFT semi-local pseudopotentials (DSSP) [40]. These core potentials includesome degree of relativistic effects and are thus very useful approx-imations for heavier elements, and, furthermore, the calculation isless computationally expensive since the core electrons aredropped. Details of the calculations are described elsewhere [41].

Geometry optimizations, without restrictions, were performedusing delocalized internal coordinates. The exchange-correlationcontribution to the total electronic energy was treated in a spin-polarized generalized-gradient corrected (GGA) form with the Per-dew-Wang-91 correlation (PW91). Density functional calculationsat the GGA level are expected to give good prediction for the bondlengths and bonding energies. We chose PW91 for our study be-cause the Perdew-Wang density functional is superior to otherDFT methods for the simultaneous prediction of both moleculargeometries and vibrational frequencies of platinum(II) complexes[42]. The vibrational frequencies were calculated and these wereused to calculate the zero-point corrections to the energies andto confirm that the structures are minima on the potential energysurface, i.e. all the vibrational frequencies are real. Free valencesand bond orders were calculated using Mayer’s procedure [43].

2.2. Experimental details

Oxaliplatin was purchased from the Sigma Chemical Companyand used as such without further purification. The FT-IR spectrum(4000–400 cm�1) was recorded on a Perkin Elmer FT-IR Spectrom-eter Spectrum 2000. High quality KBr was used as the dispersalmedium.

Fig. 1. Chemical structures of oxaliplatin and cisplatin.

3. Results and discussion

3.1. Conformers of DACH

The spectator ligand DACH plays an important role in determin-ing the higher biological activity of oxaliplatin compared to

cisplatin. Besides imparting lipophilic character to the complex,DACH is expected to display a wide range of possible conformersbecause of the presence of the cyclohexane ring. Investigationson this type of chiral complexes showed that the trans isomertrans-l (trans-(�)-1R,2R) is more efficacious than the correspondingtrans-d (trans-(+)-1S,2S) and the cis-isomer (1R,2S) [17]. Thus, theactivity might be explained by speculating on the stereochemicalstructures of the complexes.

The cyclohexane ring of DACH can have various conformations,such as chair, boat or twist boat. The chair form, expected to be themost stable one, is the obvious choice for the study, but the twistboat form has also been reported to exist [44], and is higher in en-ergy than the other possible conformers in which the cyclohexanemoiety is in the chair form.

The ligand DACH has two amino groups attached to two adja-cent carbons. Each amino group may be attached at either the axialor equatorial position. The chair form of DACH may exist in threeconformers:

(i). cis-Cyclohexyl-1,2-diamine [cis-(1R,2S)], in which one aminegroup is axial (a) and the other is equatorial (e).

(ii). trans-d-Cyclohexyl-1,2-diamine [trans-d (1S,2S)], in whichboth the amine groups are in equatorial position, and

(iii). trans-l-Cyclohexyl-1,2-diamine [trans-l (1R,2R)], in whichboth the amine groups are in equatorial position.

The (ii) and (iii) forms are optical isomers of each other.In addition to these three isomers, we have also incorporated

another form, in which the cyclohexane ring is in a twist boat con-formation, in our studies.

3.2. Relative energies

In oxaliplatin, platinum is bound on one side to the two aminenitrogens of the large 1,2-diaminocyclohexane (DACH) ligand andto the other side by two oxalate oxygens in a chelating fashion(see Fig. 1). Taking the various conformers of the cyclohexane ringin turn, the energies of the complexes were determined, and therelative energies are listed in Table 1.

As expected, the diaxial conformation of DACH is not preferredin the complex (relative energy = 36.9 kcal/mol), and the equilibriabetween the diaxial and diequatorial conformations for the transisomers favor the latter. This is due to the increased steric hin-drance of axial locations. The relative energy of 6.6 kcal/mol ob-tained for the twist boat form proves theoretically the possibilityof existence of the complex with DACH in the twist boat form athigher temperatures. No such complex has been experimentallyreported as yet.

Both the trans forms are most stable. However, a close inspec-tion of the relative energy values for trans-(S,S), trans-(R,R) andcis-(R,S) isomers reveals that they all co-exist at room temperature,as the energy difference between them is significantly less than5 kcal/mol. Of these, trans-d (S,S) and trans-l (R,R) deserve specialmention as they are optical isomers and the energy difference

Table 1DFT relative energies (kcal/mol) of the various isomers of oxaliplatin

DACH conformation Relative energy

trans ee (S,S) 0.0a

trans ee (R,R) 0.3cis ae (R,S) 1.1Twist boat form 6.6

a ZPVE corrected energy: �576828.2 kcal/mol.

Fig. 2. Structures of four oxaliplatin isomers. Colour code: H-white, C-grey, N-blue,O-red, and Pt-Prussian blue. (For interpretation of the references to colour in thisfigure legend, the reader is referred to the web version of this article.)

P. Tyagi et al. / Polyhedron 27 (2008) 3567–3574 3569

between them is negligibly small. On the basis of these calcula-tions, we expect both isomers to have almost an equal populationat room temperature.

Our expectations are borne out by the experimental results. Inan effort to solve the structure of the active species of oxaliplatin,an X-ray structural investigation was carried out on oxaliplatin[45]. The complex was prepared by reacting the enantiomericallypure isomer trans-l (trans-(�)-1R,2R-diaminocyclohexane (DACH))and the platinum salt K2[PtCl4] in H2O. They found that the oxa-liplatin which was isolated does not consist only of the desiredisomer, but a mixture of both the trans-l and trans-d isomers.No retention of optical isomerism was observed despite the factthat the enantiomerically pure DACH ligand was utilized,although it had been earlier reported [46] that only the absoluteconfiguration of the trans-l-DACH ligand exists in the platinumcomplex.

The Pt complexes of the three isomers [cis-(R,S), trans-l (R,R)and trans-d (S,S)] interact differently with DNA. It has been shownthat the trans-l (R,R) isomer of oxaliplatin is the most effectiveagainst cisplatin-sensitive and cisplatin-resistant cancer cell lines[17].

Proteins that discriminate between cisplatin–DNA adducts andoxaliplatin-DNA adducts are thought to be responsible for the dif-ferences in tumor range, toxicity, and mutagenicity of these twoimportant chemotherapeutic agents. However, the structural basisfor differential protein recognition of these adducts has not beendetermined and could be important for the design of more effectiveplatinum anti-cancer agents. Recently, a lot of work has been de-voted to understanding the differences between cisplatin and oxa-liplatin binding to DNA [47–50]. Several significant conformationaldifferences were observed between the cisplatin–GG adduct andthe oxaliplatin–GG adduct [49,50]. It has also been demonstrated[51,52] that the chirality at the carrier ligand of oxaliplatin can af-fect its biological effects.

Thus, the stereochemistry of the carrier amine ligands of cis-platin analogues can modulate their anti-cancer and mutagenicproperties. The significance of this finding is also reinforced bythe fact that, in general, interstrand cross-links formed by variouscompounds of biological significance result in greater cytotoxicitythan is expected for monofunctional adducts or other intrastrandDNA lesions [53]. Therefore, the unique properties of the inter-strand cross-links of oxaliplatin are at least partly responsible forthis drug’s unique antitumor effects.

3.3. Optimized geometries

Geometry optimizations were performed on all possible con-formers. The isomers differ in the orientations related to the cyclo-hexane ring of the DACH ligand. As far as the coordination sphere isconcerned, all the isomers exist in the square planar geometry. Thisis expected, as Pt(II) compounds show stability in this form only[54]. The oxalate group is not only planar in itself but also in planewith the coordination sphere. All the optimized structures areshown in Fig. 2.

The calculated bond lengths and bond angles for the trans-l(trans-(�)-1R,2R) isomer are listed in Table 2 along with the corre-

sponding experimental data [46] obtained from single crystal X-ray analysis of oxaliplatin. The numbering scheme used in theseoptimized structures is displayed below (Scheme 1):

Table 2Optimized geometry of the trans-R,R form (bond lengths in Ångstroms; bond angles in degrees)

Parametersa Calculated Experimentalb Parametersa Calculated Experimentalb

N1–Pt 2.10 2.06 N2PtN1 82.4 83.8N2–Pt 2.11 2.04 C7O1Pt 112.6 112.0O1–Pt 2.01 2.01 C8O2Pt 112.7 141.0O2–Pt 2.01 2.04 C8C7O1 115.4 122.0N1–C1 1.49 1.54 O3C7O1 122.5 124.0N2–C2 1.49 1.54 O3C7C8 122.2 114.0O1–C7 1.34 1.21 C7C8O2 122.1 110.0O2–C8 1.34 1.32 O4C8O2 122.7 124.0C7–C8 1.56 1.56 O4C8C7 122.1 125.0C7–O3 1.22 1.29 C1N1Pt 108.6 107.0C8–O4 1.22 1.19 C2N2Pt 109.3 106.0C1–C2 1.54 1.49 C2C1N1 108.1 107.0C1–C6 1.53 1.57 C6C1N1 113.9 105.0C2–C3 1.53 1.51 C6C1C2 111.4 111.0C6–C5 1.53 1.61 C1C2N2 108.2 103.0C3–C4 1.53 1.50 C3C2N2 113.7 113.0C5–C4 1.53 1.53 C3C2C1 111.4 111.0O2PtO1 84.2 82.5 C5C6C1 111.0 106.0N1PtO1 96.8 96.0 C4C3C2 111.1 111.0N2PtO1 179.1 175.6 C4C5C6 111.3 112.0N1PtO2 178.2 169.7 C5C4C3 111.3 111.0N2PtO2 96.6 98.6

a See Scheme 1 for numbering.b From Ref. [46].

C5

C4C3 C2

C1C6

NH2

NH2

PtO1

C7

O2 C8

O3

O4

H1

O5

H2

Scheme 1. Structure and numbering scheme of [Pt(C2O4)(DACH)] � H2O.

3570 P. Tyagi et al. / Polyhedron 27 (2008) 3567–3574

Although the computed gas phase free structure cannot be com-pared with the X-ray structure for the solid state, in the absence ofany other available data we compared the structural parameters ofthe trans-(R,R) form with the X-ray crystallographic data. It isfound that the DFT results are in good agreement with experiment(Table 2).

The optimized geometries of all the investigated isomers arecompared in Table 3. Our earlier hypothesis that the difference be-tween the isomers is limited to the DACH ligand area only has beenproved by a comparison of the geometric parameters of the iso-mers (Table 3). All the isomers have very close values as far asthe oxalate ligand parameters are concerned, since the calculatedO1–Pt, O2–Pt, C7–C8, O1–C7 and O2–C8 bond lengths have very littledifference among all the isomers. In a similar fashion, all the C–Cbond lengths, except C1–C2, which is slightly higher for the cis iso-mer, have quite similar values.

The two amino groups are bound to platinum in a way to form adistorted square planar geometry of the coordinate sphere, and thePt–N bond lengths are �2.1 Å in all the complexes. Except in thetrans-(+)-S,S isomer, there is considerable difference in the N1-Ptand N2-Pt bond lengths, but the O1–Pt and O2–Pt bond lengthsare similar. The \NPtO bond angles are much larger than the\NPtN and \OPtO bond angles.

Hence we conclude that all the chair forms have almost equalthermal stability, and are much more stable than the twist boatconformer. Of the three stable chair forms, the trans-(R,R)

conformer has been found to be the most potent [17]. Thus we con-sider only the two trans-forms for further studies.

3.4. Charge population analysis

Table 4 presents the calculated Hirshfeld [55,56] and Mullikenpartial atomic charges on the various atoms in the free ligands,DACH and oxalate anion, and the two trans complexes of Pt(II).The Hirshfeld charges are considered to give a better descriptionof the charge distribution than the Mulliken charges, which showa large dependence on the basis set.

The charge of the metal ion in the free state is +2.0. It is seenthat the positive charge of the metal ion decreases in the complexto +0.18, which indicates that transfer of electrons from the ligandsto the metal ion has occurred and the coordination bonds haveformed. To see how the charge transfer occurs, we compared thevalues of partial atomic charges on the free ligand with that ofthe Pt(II) complex (oxaliplatin), in which the charge transfer takesplace.

Although there is no change in the charge density on the carbonatoms of DACH on complexation, a decrease of 0.13 e in the chargedensity on each of the two nitrogens indicates that transfer of elec-tron density to the metal ion occurs through these coordinatedatoms.

The observation from the Mulliken charges is quite the oppo-site, i.e. an appreciable increase (�0.1) in the negative charge oneach of the C3, C4, C5 and C6 carbon atoms of DACH, and practicallyno change on the nitrogen atoms.

A decrease (�0.3) in negative charge on each of the oxygens ofoxalate shows that a considerable amount of charge is transferredfrom these four oxygens to the metal ion. Simultaneously, an in-crease of about �0.1 in the positive charges of C7 and C8 is alsonoted. These observations indicate contribution of the whole ofthe ligand in the charge transfer from the oxalate ion to the metalion.

A noteworthy point is the similarity in the charge distributionsof all the isomers, signifying that electronic differences are not thereason for the differences in their reactivity.

According to Pearson’s HSAB concept, Pt2+ is a soft acid and O2�

is a soft base, while NH3 is a hard base. This indicates that there

Table 3Optimized geometry of different conformers (bond lengths in Ångstroms; bond angles in degrees)

Parametera trans-(�)-R,R trans-(+)-S,S cis-R,S cis-R,S � H2O Twist boat

Calculated Experimentalb

N1–Pt 2.103 2.100 2.097 2.094 2.011 2.097N2–Pt 2.109 2.101 2.100 2.092 2.028 2.100O1–Pt 2.009 2.007 2.009 2.012 2.021 2.010O2–Pt 2.008 2.009 2.009 2.012 2.033 2.011N1–C1 1.493 1.493 1.497 1.498 1.500 1.495N2–C2 1.493 1.494 1.497 1.500 1.506 1.495O1–C7 1.336 1.336 1.335 1.329 1.302 1.334O2–C8 1.335 1.335 1.335 1.329 1.282 1.335C7–C8 1.555 1.556 1.557 1.555 1.563 1.557C7-O3 1.217 1.217 1.218 1.221 1.206 1.218C8–O4 1.218 1.218 1.217 1.221 1.228 1.217C1–C2 1.535 1.537 1.542 1.540 1.530 1.534N2PtN1 82.4 82.6 82.5 82.6 83.8 82.3O2PtO1 84.2 84.2 84.2 84.2 82.8 84.1N1PtO1 96.8 96.3 96.6 96.6 95.5 96.1N2PtO2 96.6 96.9 96.7 96.6 98.0 97.5N1PtO2 178.2 179.4 179.1 179.0 177.4 179.6N2PtO1 179.1 178.5 178.7 179.1 178.9 178.3O3–H1 2.149O4–H2 2.140

a See Scheme 1 for the numbering of atoms.b From Ref. [57].

Table 4The calculated Hirshfeld partial charges (Mulliken charges are in parentheses) of the optimized complexes and the ligands (DACH and oxalate ion)

Atom* Free DACH Free oxalate trans- (R,R) trans- (S,S) cis-(R,S) cis- (R,S) � H2O

C1 0.03 (0.09) 0.03 (0.05) 0.03 (0.05) 0.03 (0.03) 0.03 (0.03)C2 0.03 (0.09) 0.03 (0.05) 0.03 (0.05) 0.03 (0.04) 0.03 (0.04)C3 �0.06 (�0.07) �0.06 (�0.17) �0.06 (�0.17) �0.06 (�0.17) �0.06 (�0.17)C4 �0.05 (�0.06) �0.05 (�0.17) �0.05 (�0.17) �0.05 (�0.18) �0.05 (�0.18)C5 �0.05 (�0.06) �0.05 (�0.17) �0.05 (�0.17) �0.05 (�0.17) �0.05 (�0.17)C6 �0.06 (�0.07) �0.06 (�0.17) �0.06 (�0.17) �0.06 (�0.18) �0.06 (�0.18)N1 �0.22 (�0.39) �0.09 (�0.38) �0.09 (�0.38) �0.09 (�0.37) �0.09 (�0.37)N2 �0.22 (�0.39) �0.09 (�0.38) �0.09 (�0.38) �0.09 (�0.39) �0.09 (�0.39)C7 0.05 (0.36) 0.13 (0.45) 0.13 (0.45) 0.13 (0.45) 0.14 (0.46)C8 0.05 (0.36) 0.13 (0.45) 0.13 (0.45) 0.13 (0.45) 0.14 (0.46)O1 �0.53 (�0.68) �0.25 (�0.51) �0.25 (�0.51) �0.25 (�0.51) �0.25 (�0.51)O2 �0.53 (�0.68) �0.25 (�0.51) �0.25 (�0.51) �0.25 (�0.51) �0.25 (�0.51)O3 �0.53 (�0.68) �0.27 (�0.39) �0.27 (�0.39) �0.27 (�0.39) �0.24 (�0.41)O4 �0.53 (�0.68) �0.27 (�0.39) �0.27 (�0.39) �0.27 (�0.39) �0.24 (�0.42)Pt 0.18 (0.22) 0.18 (0.22) 0.18 (0.22) 0.20 (0.23)H1 0.11 (0.28)H2 0.10 (0.28)O5 �0.35 (�0.58)

* See Scheme 1 for the numbering of atoms.

P. Tyagi et al. / Polyhedron 27 (2008) 3567–3574 3571

should be stronger interaction between Pt2+ and O2� than betweenPt2+ and the NH2 groups of DACH. Our calculations are in accordwith this principle, as there is a transfer of only 0.6 e from DACHto Pt while 1.2 e is transferred by oxalate. This shows that a totalcharge of �1.8 e is transferred to Pt(II) during the complexation,and the net charge on Pt(II) reduces to �0.2 e. The electronic con-figuration of platinum in the complex is: [core]5s26s0.925p5.996p0.355d8.53. This shows that, out of the total chargedensity donated by the ligands to Pt2+, approximately one electrongoes to the 6s orbital, and approximately one-half to 5d, and therest of the electron density is gained by the 6p orbital.

3.5. Bond order analysis

We calculated the Mayer bond orders for all the bonds of thecomplex and the results are produced in Table 5.

Table 5 shows that there is practically no difference in thebonding pattern of the two complexes. The N–Pt and O–Pt bond or-ders are around 0.6 and 0.7, respectively, which indicates covalent

character in the bonds. Reduction in the bond order of O1–C7 andO2–C8 from 1.6 to 1.15 shows that charge has been carried awayfrom these bonds. The C7–O3 and C8–O4 bonds regain their doublebond character which was lost in the free oxalate ion due to reso-nance. However, there is a slight increase in the C7–C8 bond orderof the oxalate group. The N–C and C–C bond orders of the DACH li-gand decrease due to electron density transfer from these bonds.

We also examined the ligand orbitals in free DACH, oxalate ionand complex. Fig. 3a gives a plot of the highest occupied molecularorbital (HOMO) of DACH. This orbital has an energy of �4.71 eV. Itis clear that the HOMO is centered on the nitrogens, which beingthus electron rich, can donate charge density to the vacant metalion orbital.

Fig. 3b gives the plot of the HOMO of the oxalate ion. In oxalate,as expected, the HOMO comprises the four oxygen atoms equally.The energy of this orbital is 6.26 eV.

Fig. 3c shows the plot of the HOMO of oxaliplatin. It can be seenfrom the plot of the HOMO of the complex that not only are theoxalate ligand orbitals retained, but the metal d orbitals also

Table 5Calculated Mayer bond orders for DACH, oxalate ion and the isomers of oxaliplatin

Bond* Free DACH Free oxalate trans- (R,R) trans- (S,S) cis- (R,S) cis- (R,S) � H2O

N1–Pt 0.61 0.61 0.62 0.63N2–Pt 0.61 0.61 0.62 0.63O1–Pt 0.70 0.69 0.69 0.68O2–Pt 0.70 0.70 0.69 0.68N1–C1 1.01 0.89 0.89 0.91 0.90N2–C2 1.01 0.89 0.89 0.88 0.88C1–C2 0.97 0.95 0.95 0.96 0.96O1–C7 1.61 1.15 1.15 1.15 1.18O2–C8 1.61 1.15 1.15 1.15 1.17C7–O3 1.61 1.90 1.90 1.90 1.85C8–O4 1.61 1.90 1.90 1.90 1.85C7–C8 0.90 0.93 0.93 0.93 0.93O3���H1 0.04O4���H2 0.04

* See Scheme 1 for the numbering of atoms.

Fig. 3. Plot of HOMOs of (a) 1,2-diaminocyclohexane (DACH), (b) oxalate, (c) Pt(II)complex (oxaliplatin).

3572 P. Tyagi et al. / Polyhedron 27 (2008) 3567–3574

become part of the HOMO for the square planar Pt(II) complex,showing an interaction between oxalate and Pt(II) orbitals. The en-ergy of the HOMO of the Pt(II) complex is �4.68 eV. It is interestingto note that the HOMO of the other ligand (DACH) is not a part ofthe HOMO of the complex. This can be explained quite conve-niently by comparing the energies of the HOMOs of the two li-gands. Oxalate ion, being negatively charged, has a high energyHOMO, which can contribute electron density to Pt2+, formingthe complex whose HOMO lies above the HOMO of DACH.

3.6. Vibrational analysis

The FT-IR spectrum of oxaliplatin was recorded in the range400–4000 cm�1. The detailed list of calculated frequencies andthe corresponding intensities, as well as the experimental FT-IR

and the corresponding computed infrared absorption spectrumare given as Supplementary data. The spectra have a very compli-cated pattern, as each band corresponds to mixed vibrations.

3.6.1. Pt–Ligand VibrationsThe platinum–ligand vibrations are expected to occur in a very

low frequency range (below 600 cm�1). In the process of assigningthe frequencies obtained from the theoretical IR spectra, we foundthat the band at 551 cm�1 corresponds to the symmetric stretchingvibration, ms(Pt–N). Calculations have also revealed that the fre-quency of the asymmetric Pt–N stretching vibration ma(Pt–N),which is at 559 cm�1, is very close to ms(Pt–N). It is clear that boththe symmetric and asymmetric Pt–N stretching vibrations in oxa-liplatin contribute to a strong band observed in the experimentalFT-IR spectrum at 573 cm�1. The frequency of Pt–N stretching inoxaliplatin is higher in comparison to that in carboplatin and cis-platin. In the FT-Raman spectrum of cisplatin, the strongest bandobserved at 522 cm�1 and weaker band at 506 cm�1 were assignedto the symmetric and asymmetric Pt–N stretching vibrations [57],respectively. However in the FT-IR spectrum of carboplatin, thestrong band at 545 cm�1 and a weak band at 548 cm�1 were as-signed to the symmetric and asymmetric Pt–N stretching vibra-tions [58].

The theoretical results consistently indicate that the strongband at 780 cm�1 should be assigned to the asymmetric Pt–Ostretching and the band at 854 cm�1 to the symmetric Pt–Ostretching, coupled with the symmetric stretching of the C–C bondof the oxalate group. Similarly the very strong band at 808 cm�1 inthe experimental FT-IR spectrum is assigned to the symmetric Pt–O stretching. The medium intensity band at 306 cm�1 in the calcu-lated theoretical spectra is assigned to O–Pt–O bending. The bandin the range of 50–173 cm�1 in the theoretically predicted IR spec-trum is assigned to the N–Pt–O bending. Similarly the band at195 cm�1 is assigned to the N–Pt–N bending. Comparison of theexperimental and theoretical spectra has revealed that both thepositions and intensities of the bands are well predicted by thePW-91 method.

3.6.2. Oxalate group vibrationsIn the calculated theoretical spectrum, the very weak band at

339 cm�1 is assigned to the O–C@O in-plane bending. A strongband at 433 cm�1 is assigned to the O–C@O out-of-plane bending.The strong band at 1142 cm�1 in the calculated theoretical spec-trum is assigned to the asymmetric C–O stretching. However, verystrong bands at 1282 cm�1 and 1284 cm�1 are assigned to the Pt–Osymmetric stretching, coupled with the C–C stretching. The bandfor C–O stretching in the experimental FT-IR occurs at 1226 cm�1.

Fig. 4. Optimized structure for Pt(C2O4)(cis-DACH)] � H2O.

P. Tyagi et al. / Polyhedron 27 (2008) 3567–3574 3573

In the region 1550–1700 cm�1, the m(C@O) stretching vibrationsand the NH2 degenerate deformations, dd(NH2) are expected to oc-cur. In the FT-IR spectrum of oxaliplatin, two closely lying strongbands at 1662 and 1700 cm�1 are undoubtedly due to the C@Ostretching vibrations; however, in the theoretical spectrum thebands at 1723 and 1741 cm�1 are assigned to the C@O stretchingvibrations.

3.6.3. DACH group vibrationsThe diaminocyclohexane ring vibrations are further discussed

in two subsections:

3.6.3.1. Cyclohexane ring vibrations. In the calculated FT-IR spec-trum of oxaliplatin, the strong band at 1039 cm�1 is assigned tothe C–N stretching vibrations; similarly, in the theoretical spec-trum the very strong band at 1012 cm�1 is assigned to the C–Nstretching. In the theoretically calculated spectrum the bands at1026–1062 cm�1 are assigned to the characteristic m(C–C) stretch-ing of the cyclohexane ring, similarly the strong band at 1106 cm�1

corresponds to the symmetric stretching of the C–C bond of thetwo carbons attached to nitrogens.

The scissoring vibrations of the methylene group in oxaliplatinare assigned to the bands observed at 1444 and 1456 cm�1 in thecalculated theoretical spectrum. Similar results are recorded forthe theoretical spectrum of carboplatin in which the scissoringvibrations of methylene group were observed at 1436 and1463 cm�1 [58]. The band in the range of 2954–3058 cm�1 is as-signed to the C–H stretching vibration in the calculated theoreticalspectrum; similar bands at 2928 and 3085 cm�1 appear in theexperimental FT-IR spectrum.

3.6.3.2. Amino group vibrations. The strong band at 3085, 3158 and3212 cm�1 in the experimental FT-IR spectrum and the bands inthe range of 3364–3460 cm�1 in the calculated theoretical spec-trum are assigned to the N–H stretching vibrations of the aminogroups of oxaliplatin.

The degenerate deformation vibration of ammonia gives astrong band at 1610 cm�1 in the FT-IR spectrum; similarly, weget two strong bands at 1609 and 1620 cm�1 which correspondto wagging of NH2. The symmetric deformation vibrations of NH2

are coupled with the degenerate deformation mode (twisting andwagging vibrations) of the methylene group and generate the med-ium intensity bands at 1444 and 1456 cm�1 in the calculated the-oretical spectrum. However, in the experimental FT-IR spectrum,these vibrations give a very strong band at 1377 cm�1. No peakcorresponding to NH2 rocking vibrations is found in the FT-IR spec-trum since, as shown by theoretical calculations, NH2 rockingvibrations give rise to the bands at 659 and 697 cm�1 of negligibleintensity.

3.7. Oxaliplatin monohydrate

Al-Allaf et al. [59] synthesized crystals of oxaliplatin for X-raydetermination. It was found that, although the complex was pre-pared starting from the enantiomerically pure isomer trans-l(1R,2R) as confirmed by X-ray analysis [45], it does not consist ofthe desired (1R,2R) isomer, but rather the (1R,2S) isomer. Thisencouraged them to prepare the platinum complex of the cis iso-mer of the DACH ligand in order to study its behavior from theX-ray point of view and to compare the results with those of theisomeric complex [Pt(C2O4){(1R,2R)-DACH}] reported previously[46]. It was seen that the [Pt(C2O4)(cis-DACH)] complex crystallizeswith one molecule of water as [Pt(C2O4)(cis-DACH)] � H2O in well-shaped colorless crystals.

The water molecule is reported to be placed at the oxalate end,in plane with the oxalate group and oriented to have two H-bonds

with the two non-bound oxygens of the oxalate ligand [59]. Wetherefore restricted our studies to this position of the water mole-cule. The hydrated molecule thus has following structure:

3.7.1. Optimized geometriesThe optimized geometry of cis-(R,S) was taken as calculated ear-

lier. The water molecule is placed in such a way that the twohydrogen of the water molecule are in close contact with thetwo carbonyl oxygens of the oxalate ring, as has been previouslyreported [59], and geometry optimization was performed on theresulting structure. The numbering scheme and the optimizedstructure of [Pt(C2O4)(cis-DACH)] � H2O are shown in Figs. 2 and 4.

The calculated bond lengths and bond angles for the [Pt(C2O4)(cis-DACH)] � H2O isomer are listed in Table 3, together with thecorresponding experimental data obtained from single crystalX-ray analysis of [Pt(C2O4)(cis-DACH)] � H2O [59] to check theaccuracy of our calculation.

It is found that the DFT results are in good agreement with theexperimental data within the error range, considering that the for-mer are obtained for the gas phase and the latter are for the solidstate.

To gauge the effect of hydrogen bonding on the geometry andstabilization of the complex we compared the geometric parame-ters of the hydrated complex with the anhydrous complex. It isfound that there is no major change in the geometric parametersof DACH in the hydrated and anhydrous [Pt(C2O4)(cis-DACH)]forms of the complex.

A slight change is noted in the geometric parameters of thecoordination sphere. In the hydrated form, the equatorial N–Ptbond is shorter by 0.003 Å, while for the axial N–Pt bond thischange is 0.008 Å. The O–Pt bond gets elongated by 0.003 Å. Nochange is noted in the equatorial N1–C1 bond, but the equatorialN2–C2 bond increases by 0.003 Å.

The effect on the bond lengths of the oxalate ring in the hy-drated form is found to be larger, as expected. The O1–C7 andO2–C8 bond lengths decrease by 0.006 Å, while an increase in bondlength by 0.004 Å is noted in case of the C7–O3 and C8–O4 bonds.The distance of 2.1 Å between the carbonyl oxygen of the oxalategroup and the hydrogens of water, indicates hydrogen bonding be-tween the two, as this is much smaller than the sum of the van derWaals radii of oxygen and hydrogen (2.72 Å).

3.7.2. Charge population analysisTable 4 represents the calculated Hirshfeld partial atomic

charges in [Pt(C2O4)(cis-DACH)] � H2O and anhydrous [Pt(C2O4)(-cis-DACH)]. No significant change is noted in the charge densitieson the atoms of the DACH ring in the anhydrous and hydrated ciscomplex. A decrease of 0.03 in the negative charge on O3 and O4

is noted, showing the decrease in electron density at these twooxygens on hydration, as these two oxygens are directly involvedin hydrogen bonding.

3574 P. Tyagi et al. / Polyhedron 27 (2008) 3567–3574

3.7.3. Bond order analysisWe calculated the Mayer bond order for the bonds in [Pt(C2O4)

(cis-DACH)] � H2O, and the results are compared with those in theanhydrous [Pt(C2O4)(cis-DACH)] complex to find out the structuraldifference caused by the hydrogen bonding on the metal ligandbinding. Table 5 shows the calculated bond orders for [Pt(C2O4)(cis-DACH)] � H2O and the anhydrous complex.

Compared to the anhydrous complex, the Pt–N bond strength-ens slightly, while the Pt–O bond becomes slightly weaker. Thus,coordination with a water molecule increases the capacity of oxa-late as a leaving group. No significant change is noted in the bondorders of the bonds involved in the DACH ring for the hydrated andanhydrous forms.

The O1–C7 and O2–C8 bond orders increase by about 0.02. A ma-jor change is noted in case of the C7–O3 and C8–O4 bonds. The bondorders here decrease by about 0.05, which indicates the weakeningof these bonds as a result of the transfer of electrons away fromthem, due to hydrogen bonding with water.

The O3���H1 and O4���H2 bond orders are expected to be zero, buta value of 0.04 is calculated, which indicates that hydrogen bondshave formed.

4. Conclusions

We have performed density functional calculations at the GGA-PW91 level in order to gain an understanding into the structure ofthe third generation anti-cancer drug, oxaliplatin. We have paidparticular emphasis on the conformation of the DACH ligand andhave found that the trans forms of the ligand are more stable thanthe cis form. However, of the two equally stable enantiomers, thetrans-l (1R,2R) one is found to be more potent biologically. Sincevery minor differences are observed in the electronic structuresof the two enantiomers, their difference in activity may be attrib-uted to the chiral recognition of the ligand by DNA. The calculatedvibrational frequencies are in good agreement with our experi-mental FT-IR spectrum. It has been observed that the trans-d(1S,2S) and the cis-(1R,2S) isomers are less active than the trans-l(1R,2R) isomer, but have similar activities. Calculations were alsoperformed on the cis isomer and its monohydrate, experimentaldata on whose geometry is known. Comparison between the theo-retically predicted geometries and the experimental ones yieldedgood correspondence, validating our methodology, although thesecalculations pertain to the gas phase, and the experimental dataare for the solid state, in which crystal-field effects may affectthe relative energies of the conformers and the geometries. Inthe biological environment, hydrogen bonding with the solvent isalso expected to affect the relative energies of the conformers,but the good correspondence between our calculations and exper-imental data suggest that crystal packing and hydrogen bondingeffects are negligible.

Acknowledgements

The authors thank University of Delhi’s ‘‘Scheme to StrengthenR&D Doctoral Research Programme by Providing Funds to Univer-sity Faculty”. One of the authors (PG) also thanks the Council of Sci-entific and Industrial Research, New Delhi, for a Senior ResearchFellowship.

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